Pressure control system for fuel cell gas spring

ABSTRACT

In a fuel cell assembly, a passive gas spring is disposed between the stack and the supporting structure for maintaining compressive force on the stack and manifold seal. As variation in temperature of the assembly and structure causes dimensional changes therein, the pressure within the gas spring also changes accordingly. The gas spring is provided with inlet and outlet check valves, the outlet check valve opening to expel air when internal spring pressure reaches a predetermined upper pressure limit, and the inlet check valve opening to admit air when the internal spring pressure falls below a predetermined lower pressure limit in a currently preferred embodiment, the outlet check valve allows exit of gas from the gas spring at pressures exceeding 5 psig, to prevent rupture of the gas spring, and the inlet check valve allows entrance of gas into the gas spring as the spring cools following use.

TECHNICAL FIELD

[0001] The present invention relates to hydrogen/oxygen fuel cells; moreparticularly, to method and apparatus for applying a compressive load toa fuel cell stack assembly and supply manifold during manufacture andfor maintaining a compressive load thereupon during use; and mostparticularly, to method and apparatus for automatically venting andrefilling a gas spring during use thereof to maintain pressures withinthe spring within a predetermined range.

BACKGROUND OF THE INVENTION

[0002] Fuel cells which generate electric current by controllablycombining elemental hydrogen and oxygen are well known. In one form ofsuch a fuel cell, an anodic layer and a cathodic layer are deposited onopposite surfaces of a permeable electrolyte formed of a ceramic solidoxide. Such a fuel cell is known in the art as a “solid oxide fuel cell”(SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowedalong the outer surface of the anode and diffuses into the anode.Oxygen, typically from air, is flowed along the outer surface of thecathode and diffuses into the cathode where it is ionized. The oxygenions diffuse through the electrolyte and combine with hydrogen ions toform water. The cathode and the anode are connected externally throughthe load to complete the circuit whereby electrons are transferred fromthe anode to the cathode. When hydrogen is derived from “reformed”hydrocarbons, the reformate gas includes CO which is converted to CO₂ atthe anode. Reformed gasoline is a commonly used fuel in automotive fuelcell applications.

[0003] A single cell is capable of generating a relatively small voltageand wattage, typically between about 0.5 volt and about 1.0 volt,depending upon electrical load, and less than about 2 watts per cm² ofcell surface. Therefore, in practice it is usual to stack together, inelectrical series, a plurality of cells.

[0004] Because each anode and cathode must have a free space for passageof gas over its surface, the cells are separated by perimeter spacerswhich are vented to permit flow of gas to the anodes and cathodes asdesired but which form seals on their axial surfaces to prevent gasleakage from the sides of the stack. The perimeter spacers includedielectric layers to insulate the interconnects from each other.Adjacent cells are connected electrically by “interconnect” elements inthe stack, the outer surfaces of the anodes and cathodes beingelectrically connected to their respective interconnects by electricalcontacts disposed within the gas-flow space, typically by a metallicfoam which is readily gas-permeable or by conductive filaments. Theoutermost, or end, interconnects of the stack define electric terminals,or “current collectors,” which may be connected across a load.

[0005] A complete SOFC assembly typically includes auxiliary subsystemsfor, among other requirements, generating fuel by reforminghydrocarbons; tempering the reformate fuel and air entering the stack;providing air to the hydrocarbon reformer; providing air to the cathodesfor reaction with hydrogen in the fuel cell stack; providing air forcooling the fuel cell stack; providing combustion air to an afterburnerfor unspent fuel exiting the stack; and providing cooling air to theafterburner and the stack. These various subsystems typically are matedvia mounting to an integrating manifold. A complete SOFC assembly alsoincludes appropriate piping and valving, as well as a programmableelectronic control unit (ECU) for managing the activities of thesubsystems simultaneously.

[0006] During assembly of a fuel cell stack, a compressive load must bemaintained during high-temperature sintering of the stack assemblyseals. Desirably, a light compressive load is maintained after thesintering process to ensure the integrity of the glass seals to themanifold during assembly and also afterwards during use of a finishedfuel cell assembly.

[0007] The stack assembly is made from a variety of metallic andnon-metallic materials, and the supporting structure fastening the stackto its manifold is constructed of, typically, a bolting material capableof withstanding high temperatures. At operating temperature, typicallyaround 800° C., thermal growth of the stack does not match thermalgrowth of the bolting material because of differences in thermalexpansion coefficients, which mismatch can result in loss of compressiveload against the various seals.

[0008] To compensate for this mismatch, it is known to use springswithin the assembly. However, high operating temperatures can affecttemper of spring materials, resulting in load failure. Further, springconstants typically diminish with increase in temperature, conditionsunder which an increase in spring force is desirable to compensate forincreasing mismatch.

[0009] Further, a fuel cell assembly may comprise a plurality of fuelcell stacks disposed side-by-side within a single supporting structure,and different stacks may vary in height at different temperatures.

[0010] Therefore, a gas-filled pillow or gas spring may be used withinthe assembly, the gas being thermally expandable to ensure excellentsealing of the elements as the temperature of a fuel cell assembly isincreased during seal sintering and operation.

[0011] A problem arises with gas springs, however, in that thedifferential between ambient temperature, e.g., 20° C., and sintering oroperating temperature, e.g., 800-1000° C. is very large. A gas springfilled to ambient pressure (0 psig) at ambient temperature exhibitspressures at elevated temperatures far in excess of what is needed toprovide reliable sealing, e.g., 5 psig. Further, such high pressure canbe sufficient to cause rupture of the gas spring and consequent failureof the fuel cell assembly.

[0012] What is needed is a means for providing a compressive load to afuel cell assembly at ambient and elevated temperatures to compensatefor mismatches in the heights of multiple stacks and for the differencein thermal expansion between the stacks and the supporting structure,and further, means for maintaining such compressive load within apredetermined pressure range.

[0013] It is a principal object of the present invention to compress afuel cell assembly automatically within a predetermined range ofpressures under all required temperature conditions during manufactureand use.

SUMMARY OF THE INVENTION

[0014] Briefly described, in a fuel cell assembly comprising one or morefuel cell stacks and a supporting structure, a passive gas spring isdisposed between the stacks and the supporting structure for maintainingcompressive force on the stack and manifold seals. The spring includes amembrane formed of a metal alloy stable at the operating temperaturesrequired of the fuel cell assembly. As variation in temperature of theassembly and structure causes dimensional changes therein, the pressurewithin the gas spring also changes accordingly. The gas spring isprovided with inlet and outlet check valves, the outlet check valveopening to expel air when internal spring pressure reaches apredetermined upper pressure limit, and the inlet check valve opening toadmit air when the internal spring pressure falls below a predeterminedlower pressure limit. Internal pressure is thereby automaticallycontrolled within a predetermined range of operating pressures, thusmaintaining a compressive load on the fuel cell stack over the fullrange of temperature variation.

[0015] In a currently preferred embodiment, the outlet check valveallows exit of gas from the gas spring at, preferably, pressuresexceeding 5 psig, to prevent rupture of the gas spring; and the inletcheck valve allows entrance of gas into the gas spring as the springcools following use. Entrance air may be pressurized in known fashion tomaintain a slight positive pressure in the spring, or the inlet checkvalve may open in response to ambient air pressure, so that the gasspring begins a thermal cycle at 0 psig.

[0016] An advantage of the present invention is that a suitable maximumgas pressure may be provided in a gas spring at elevated temperaturewithout concomitantly creating a negative gas pressure in the spring atambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0018]FIG. 1 is a schematic drawing, showing a gas spring having inletand exhaust check valves, in accordance with the invention;

[0019]FIG. 2 is an elevational cross-sectional view of a portion of agas spring, showing incorporation of an inlet check valve; and

[0020]FIG. 3 is an elevational cross-sectional view of a fuel cell stackassembly including the gas spring shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Referring to FIGS. 1 and 2, in accordance with the invention, anovel gas spring 10 comprises a closed frame element 12 having axis 14.Element 12 preferably is formed in a trough shape to resist radialdeformation under load and may be radially concave or convex to equaleffect. Frame element 12 includes first and second axial surfaces 16,18to which at least one flexible membrane, and preferably first and secondmembranes 20,22, respectively, is continuously attached as by laserwelding 24 to form a flattened pillow defining a chamber 26. Preferably,membranes 20,22 are formed of a flexible high-temperature metal alloy,for example, Haynes 160, 214, 230, 825, or 901; a Hasteloy; or InconelDS, 625, or 718. Preferably, membranes 20,22 are between about 0.005inch and 0.010 inch in thickness. Chamber 26 is filled with a gas 28,preferably air, which may be installed in known fashion at any desiredpressure and temperature above or below atmospheric and ambient for anyspecific application; one atmosphere at ambient temperature is currentlypreferred for fuel cell uses.

[0022] Gas spring 10 further includes a first check valve 30 exemplarilydisposed in element 12 via a first threaded opening therein. Valve 30thus communicates between the exterior 32 of spring 10 and chamber 26.Valve 30 may comprise any convenient check valve design as may be knownin the art of check valves.

[0023] As shown herein, valve 30 includes a tapered valve seat 34, acheck ball 36, and a spring 38 retained by cage 40. Valve 30 is orientedto admit gas into chamber 26 when pressure in exterior 32 exceeds thecombined pressure of gas 28 in chamber 26 plus the force of spring 38.In a currently preferred embodiment, the force of spring 38 is verynearly zero, about 0.1 psig, sufficient to maintain ball 36 in place onseat 34; thus, the lower operating limit of gas spring 10 is about 1atmosphere, or 0 psig. Of course, if higher minimum pressures aredesired, a source 42 of gas at the desired pressure may be connected tovalve 30, as shown in phantom in FIG. 2.

[0024] Referring to FIGS. 2 and 3, gas spring 10 further includes asecond check valve 50 exemplarily disposed in element 12 via a secondthreaded opening therein. Valve 50 thus communicates between theexterior 32 of spring 10 and chamber 26. Valve 50 may comprise anyconvenient check valve design as may be known in the art of check valvesand preferably is substantially identical to first check valve 30. Valve50 is oriented to vent gas from chamber 26 when the pressure of gas 28in chamber 26 exceeds the predetermined force of the valve spring. In acurrently preferred embodiment for use with a fuel cell assembly, theforce of the spring is selected such that gas is vented from chamber 26at about 5 psig.

[0025] It will be obvious to one of ordinary skill in the art that firstand second check valves 30,50 may be installed, within the scope of theinvention, at any of many locations in gas spring 10, not only inelement 12 but in membranes 20,22 as well. Further, gas springs inaccordance with the invention may be formed for use in some applicationsby omitting frame element 12 entirely and directly sealing membrane 20to membrane 22 as by laser welds to form a gas-filled pillow.

[0026] Still referring to FIG. 3, gas spring 10 is beneficially employedin a solid-oxide fuel cell assembly 70 to generate axial pressure onfuel cell stack 72. As the temperature of captive gas 28 rises,increased outward pressure is exerted on element 12 and membranes 20,22in accordance with Boyle's Law, urging the membranes apart axially asshown by phantom membranes 20′,22′ in FIG. 2. When installed in assembly70, membranes 20,22 are restrained by spring holder 74 and springretaining plate 76. Thus, thermal expansion of gas spring 10 urgesspring holder 74 toward base plate 78, keeping stack 72 undercompression, and urges retaining plate 76 away from manifold 80, thuskeeping bolts 82 under tension and seal 84 under compression.

[0027] While the invention has been described by reference to variousspecific embodiments, it should be understood that numerous changes maybe made within the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

What is claimed is:
 1. A gas spring system having a spring forcevariable with temperature, comprising: a) a first membrane; b) a secondmembrane; c) means for sealing edges of said first and second membranesto define a closed chamber therebetween for capture of gas; d) firstvalve means for admitting gas to said chamber; and e) second valve meansfor exhausting gas from said chamber.
 2. A gas spring system inaccordance with claim 1 wherein said means for sealing includes directsealing of said first membrane to said second membrane to form agas-filled pillow.
 3. A gas spring system in accordance with claim 1wherein said means for sealing includes a rigid frame element disposedbetween said first and second membranes.
 4. A gas spring system inaccordance with claim 3 wherein said frame element has a trough-shapedcross section.
 5. A gas spring system in accordance with claim 4 whereinsaid trough shape is radially concave.
 6. A gas spring system inaccordance with claim 4 wherein said trough shape is radially convex. 7.A gas spring system in accordance with claim 1 wherein said first valvemeans is a check valve.
 8. A gas spring system in accordance with claim7 wherein said check valve is closed at all pressures across said valveexceeding about 0.1 psig.
 9. A gas spring system in accordance withclaim 1 wherein said second valve means is a check valve.
 10. A gasspring system in accordance with claim 9 wherein said check valve isclosed at all pressures across said valve less than about 5 psig.
 11. Afuel cell assembly, comprising: a) at least one fuel cell stack; b) asupporting structure surrounding said fuel cell stack; and c) a gasspring disposed within said assembly between said stack and saidsupporting structure, said spring including a first membrane, a secondmembrane, means for sealing edges of said first and second membranes todefine a closed chamber therebetween for capture of gas, first valvemeans for admitting gas to said chamber, and second valve means forexhausting gas from said chamber.
 12. A fuel cell assembly in accordancewith claim 11 wherein said fuel cell stack includes at least onesolid-oxide fuel cell.
 13. A fuel cell assembly comprising: a) at leastone fuel cell stack; b) a supporting structure surrounding said fuelcell stack; and c) gas spring means disposed within said assemblybetween said stack and said supporting structure, said gas spring meansdefining a closed chamber and including an inlet valve for admitting gasinto said chamber and an outlet valve for exhausting gas from saidchamber.